Coalbed Methane (CBM) is a natural gas formed by geological processes in coal seams and consists predominantly of methane, the major chemical component in natural gas. CBM is an all in one natural gas resource as it serves as the source, reservoir, and trap for a vast amount of potential natural gas. Typically, CBM can be found unexploited at relatively shallow depths, and because methane is stored in coal by a different means than conventional gas, more gas per unit volume can be recovered at these shallow depths.
Various methods have been utilized by the energy industry to extract CBM from subterranean formations. In most instances wellbores are drilled to penetrate the hydrocarbon-containing formations into sections commonly referred to as “production intervals.” A subterranean formation penetrated by a wellbore may have multiple production intervals at various depths in the wellbore. Generally, after a wellbore has been drilled to a desired depth, completion operations may be undertaken, usually involving the insertion and cementing of steel casing into the wellbore. In order to extract hydrocarbons from the coal seam, the casing and cement housing are perforated to create production intervals through which hydrocarbons can flow into the wellbore and ultimately to the surface.
To enhance hydrocarbon production, the production intervals are often stimulated by a variety of methods that have been developed and used successfully for increasing the production of CBM from coal seams. Typical stimulation operations may involve hydraulic fracturing, acidizing, fracture acidizing, or combinations thereof. Hydraulic fracturing generally includes injecting or pumping a viscous fracturing fluid into a portion of the subterranean formation at a rate and pressure such that fractures are formed or enhanced into the portion of the subterranean formation. The incident pressure causes the formation to crack which allows the fracturing fluid to enter and extend the crack further into the formation. The fractures tend to propagate as vertical and/or horizontal cracks located radially outward from the wellbore.
In such treatments, once the hydraulic pressure is released, the fractures formed will tend to close back onto themselves, possibly preventing hydrocarbon flow. To prevent this closure, a sieved round sand known as proppant can be disposed in the fractures by suspending them in the pumped fracturing fluid during at least a portion of the fracturing operation. The proppant is carried into the newly created fractures and deposited therein such that when the hydraulic pressure is released the proppant acts to prevent the fracture from fully closing and provides highly permeable conduits through which the formation fluids can be produced back to the well.
In some applications, hydraulic fracturing stages are immediately followed by the injection or pumping of an acidizing solution which can flow above the fracturing fluid and proppant deposited in the lower portion of a vertical fracture, thus having a tendency to widen and vertically extend the upper portion of a fracture. Acidizing may also initiate new fractures and clean the wellbore and fracture faces by dissolving any precipitates or contaminants due to drilling or completion fluids or cement which may be present at or adjacent the wellbore or fracture faces.
It nonetheless remains desirable to find improved methods for fracturing and stimulating new or existing subterranean coal seams. It is desirable to find methods that introduce different fracturing fluids having diverse chemical properties and methods that reduce or eliminate the need for proppants. By doing so, significant savings of time and operating expense may be accrued.